help me with JFrame and JLabel

hello!
I have been trying, with some friends help, for a couple of days now to make a "countdown" program. The main point of this program was to see a time running down in a window. I have tried to make my own window with JFrame and successed. (I have done a new label and everything) But now the problem is how to get the label updated so I can see the time "running" down. I have tested with "setText" from the main platform JLabel but I have not succesed. Maybe the foult is that I use a Calendar platform from java.util to count how much time it's left.
I realy need help with this problem.
Ty... Michal:)